In the prior art, a technique called Fast Tool Servo has been known. In Fast Tool Servo, actuators for supporting tools are driven at high speeds at frequencies many times higher than the response frequencies of the tables (movable axes) of a processing machine. The actuators are driven on the order of several hundreds hertz and include a piezoelectric element, a magnetostrictor, and a voice coil which can be operated at high speeds. Although it is difficult to operate the heavy tables of the processing machine at high speeds, it is relatively easy to minutely displace only the tools at high speeds by the actuators which can be operated at high speeds.
Fast Tool Servo is described in, for example, “Japanese Patent Laid-Open No. 2007-75915”, “Japanese Patent Laid-Open No. 2007-307663”, and “Fast Tool Servo, Yuichi Okazaki, Journal of the Japan Society for Precision Engineering, the Japan Society for Precision Engineering, 2006, Vol. 72, No. 4, pp. 422-426”.
FIG. 18 is a typical structural diagram showing a cutting device for realizing Fast Tool Servo. FIG. 19 is a typical control block diagram of the cutting device for realizing Fast Tool Servo.
As shown in FIG. 18, the operating directions (axial directions) of an actuator 101, an actuator 102, and an actuator 103 are orthogonal to one another. A tool holder 104 is disposed at the intersection of the three axial directions. The tool holder 104 has a tool 105 attached thereon. Displacement sensors 106a, 106b, and 106c measure the displacements of sensor targets 107a, 107b, and 107c attached to the tool holder 104, and generate displacement sensor signals representing the measured displacements. These displacement sensor signals represent the translational displacements of the actuators 101, 102, and 103.
The driving of the three-axis actuators 101, 102, and 103, that is, the displacements of the actuators are controlled by a controller 111 shown in FIG. 19. As shown in FIG. 19, the controller 111 is fed with position information (X coordinate, Y coordinate, Z coordinate, and B coordinate) from an ultraprecision machine (not shown). Based on the position information from the ultraprecision machine, a real-time target value arithmetic section 112 provided in the controller 111 calculates the target displacements (command values) of the three-axis actuators 101, 102, and 103. According to the command values, actuator driving amplifiers 113a, 113b, and 113c drive the actuators 101, 102, and 103. The displacement sensor signals from the displacement sensors 106a, 106b, and 106c mounted respectively on the actuators 101, 102, and 103 are inputted to the respective feedback systems of the three axes. In other words, the driving, that is, the displacement of each of the three-axis actuators 101, 102, and 103 is feedback controlled.
The following will describe the operations of the cutting device configured thus. During machining, the movable axes (X axis, Y axis, Z axis, and B axis) of the ultraprecision machine are operated based on the NC data of the ultraprecision machine, and the tool 105 minutely operated according to the command values generated based on the position information from the ultraprecision machine has a cutting edge coming into contact with a work material 108 rotating about the B axis, so that the work material 108 is machined.
The cutting device for realizing Fast Tool Servo makes it possible to minutely displace the cutting edge of the tool at high speeds, and machine a nonaxisymmetric aspheric surface (free-form surface) as if the nonaxisymmetric aspheric surface was lathed. Thus it is possible to dramatically shorten the machining time of the nonaxisymmetric aspheric surface (free-form surface).
In a typical cutting device for realizing Fast Tool Servo, however, cutting is performed in synchronization with the position coordinate (B coordinate) of a rotation axis (B axis) and the position coordinate of one of the X axis and the Y axis (one of the X coordinate and the Y coordinate) and thus causes large machining resistance as in ordinary lathing. The large machining resistance causes an elastic deformation on the actuators which support the tool. The elastic deformation on the actuators results in an error of a machined shape. Thus a typical cutting device for realizing Fast Tool Servo cannot achieve machining with higher precision.
Further, in a typical cutting device for realizing Fast Tool Servo, actuators have to be driven at frequencies not higher than the response speed of a feedback control system. To be specific, for example, in a cutting device described in “Japanese Patent Laid-Open No. 2007-75915”, an actuator has a driving frequency of about 1 kHz. Thus the rotation speed (cutting speed) of the B axis has to be set at about 100 rpm or less, though the rotation speed varies with the machining position. Generally, in lathing, machining resistance increases as a rotation speed decreases. Thus when the rotation speed is reduced, it is not possible to perform high-precision machining.
Another machining technique is called elliptical vibration cutting. In elliptical vibration cutting, vibration components are applied to a tool in two directions, the cutting edge of the tool is minutely operated according to an elliptical shape by optimally controlling a phase difference between the vibration components in the two directions, and a work material is cut by the cutting edge of the tool which is minutely operated according to the elliptical shape. This machining technique can dramatically reduce machining resistance, so that it is expected that chipping can be prevented on the tool and the life of the tool can be increased.
Elliptical vibration cutting is described in, for example, “Japanese Patent No. 3500434” and “Japanese Patent No. 3806603”.
In a typical vibration cutting device for realizing elliptical vibration cutting, however, the machining resistance can be reduced but the machining time cannot be shortened.
Although a combination of Fast Tool Servo and elliptical vibration cutting has been demanded, these techniques use completely different frequency bands in control and thus cannot be combined.
To be specific, Fast Tool Servo uses a frequency of about 1 kHz or less in control. This is because an increase in used frequency band causes a phase delay and high machining accuracy cannot be obtained. Moreover, an increase in used frequency band may cause oscillation and the like.
On the other hand, elliptical vibration cutting uses a frequency higher than 1 kHz in control. Although elliptical vibration cutting can be performed at 1 kHz or less, the machining speed is reduced and thus a frequency of 1 kHz or less is not used in the industry.
The vibration cutting device can control the number of vibrations and an amplitude but cannot control the absolute position of the cutting edge of a tool. Thus unfortunately, elliptical vibration cutting cannot obtain high accuracy of form in a long machining time. This is because when the machining time increases, the absolute position of the cutting edge of the tool is changed by a change in machining atmosphere, for example, a temperature change. For example, when machining is performed by elliptical vibration cutting over several hours as in the machining of a mold master of a light guide plate, high machining accuracy cannot be obtained.